A common rail fuel system typically comprises a common rail body in the form of a high pressure pipe which is charged to a high pressure by an appropriate high pressure fuel pump. A plurality of connection lines are connected to the common rail body via branch holes or outlets in the pipe, each connection line leading to a respective fuel injector. The central bore of the pipe therefore acts as a pressure accumulation chamber from which pressurised fuel is distributed towards the injectors.
It will be understood that during each fuel delivery cycle the common rail is subject to significant stresses caused by the high fuel pressure. In particular, the inner pressure of the rail causes high tensile stress at the peripheral edges of the openings where the bore of the pipe meets the branch holes. Over a period of time, such stress is liable to cause cracks in the vicinity of the openings and hence may give rise to fuel leakage.
Accordingly, efforts have focussed on improving the resistance of the common rail to stress fatigue. By increasing the strength of the common rail, it becomes possible to increase the acceptable cycled running pressure of the fuel supply system.
In the majority of known fuel supply systems, the common rail body is typically of circular cross-section. The simplest way to improve durability of such a rail is to increase the diameter of the body. Although this is readily achievable, any benefit in terms of enhanced durability is largely offset by the attendant material cost resulting from an increase in the quantity of steel required to produce a thicker rail.
Another approach to improving fatigue strength, as described in JP 10169527, is to use a pipe of elliptical cross-section for the common rail. Such an elliptical pipe is shaped by plastically deforming a round pipe, for example by press working or roll forming. However, the deformation process itself induces residual stresses in the common rail body. So, stresses always remain at the intersecting portion where the branch opening meets the pressure accumulation bore. Even if the tensile stresses at the intersecting portion induced by the inner pressure of highly pressurised fuel in the pressure accumulation chamber is reduced, the sum of the residual stresses and the stresses due to inner pressure is likely to be large enough still to compromise strength at the intersecting portion.
In the above arrangement, by virtue of the manufacturing process, both the pipe and the pressure accumulation chamber created by the central bore are elliptical in cross-section and pipe thickness remains substantially uniform throughout. By contrast, in US-A1-2001/0029929 a common rail pipe having a substantially circular outer cross-section and internal bore of roughly oval or flat oval cross-section is described. Although the creation of the internal bore does not in this case involve deformation of a round pipe and hence avoids residual stresses, a two stage manufacturing process is required; the first stage involving drilling to form a round bore and in the second stage the opposite sides of an inner wall of the round bore are removed by broaching or by electric discharge machining to create the oval cross-section.
It will be appreciated therefore that the common rail bodies made according to the prior art suffer from disadvantages. Accordingly, it is an object of the present invention to provide a fuel injection system having a common rail body of improved durability that may be readily manufactured.